Compositions of nanometal particles containing a metal or alloy and platinum particles for use in fuel cells

ABSTRACT

A composition of nanoparticles of metal or an alloy or having a metal and alloy core with an oxide shell in admixture with platinum particles is useful as a component for electrodes. More particularly, such composition is useful as an electrode ink for the reduction of oxygen as well as the oxidation of hydrocarbon or hydrogen fuel in a direct oxidation fuel cell, such as, but not limited to, the direct methanol fuel cell. These electrodes encompass a catalyst ink containing platinum, the nanoparticles, and a conducting ionomer which may be directly applied to a conductive support, such as woven carbon paper or cloth. This electrode may be directly adhered onto an ion exchange membrane. The nanoparticles comprise nanometer-sized transition metals such as cobalt, iron, nickel, ruthenium, chromium, palladium, silver, gold, and copper. In this invention, these catalytic powders substantially replace platinum as a catalyst in fuel cell electrooxidation and electroreduction reactions.

TECHNICAL FIELD

The present invention relates to compositions comprising nanoparticlesof a metal and/or alloy or nanoparticles comprising a metal or alloycore surrounded by an oxide shell in admixture with platinum particles.More particularly, the composition is useful for inks used to make anodeand cathode electrodes, which may be used in fuel cells.

BACKGROUND ART

Platinum is highly catalytic for hydrocarbon or hydrogen oxidation andoxygen reduction in gas diffusion electrodes for a variety of fuelcells. However, this noble metal is a rapidly depleting non-renewableresource and is consequently expensive. Current price for bulk platinumblack is $75.00/gram. The associated cost of a platinum depositedelectrode, typically loaded anywhere from 2-8 mg/cm², is widelyconsidered to be a hurdle to widespread commercialization. With thegaining demand for alternative energy sources by consumers, efficientcatalysts, especially at practical operating temperature (roomtemperature to 60° C.) must be discovered to alleviate the demand andexpense of platinum. Based on this, considerable effort is beingdedicated to find an alternative catalyst which can match or exceedplatinum's electrical performance. Method of synthesis of metalnanoparticles has been previously described in U.S. patent applicationSer. No. 10/840,409, as well as their use in air cathodes for batteriesin U.S. patent application Ser. No. 10/983,993 both of whichapplications have the same assignee as the present application. Thedisclosures of these applications are incorporated herein by reference.Platinum particles have also been prepared for fuel cell electrodes bychemical reduction on carbon.

DISCLOSURE OF THE INVENTION

Nanoparticle catalysts can be used to supplement platinum catalysts forfuel cell electrodes embodiments of the invention. Embodiments includenanoparticle catalysts of cobalt, iron, nickel, ruthenium, chromium,palladium, silver, gold, and copper and their alloys that are at leastnearly as active as platinum for the reduction of oxygen or oxidation ofhydrocarbon fuel in direct oxidation fuel cells. Various embodimentsdescribed herein discuss metal nanoparticle catalysts for directmethanol fuel cell applications, but are equally applicable to otherapplications, for example without exclusion (i) proton exchange membranefuel cells (PEMFC's), and formic acid fuel cells (FAFC's).

A first embodiment includes nanoparticles, which can comprise a singlemetal or an alloy of two or more transition metals, optionally having anoxide shell surrounding the metal or alloy core admixed or physicallyblended with platinum particles. Preferably, these platinum particlesare under one micron in size, which are classified as finely divided.Preferably, the platinum particles should be below 100 nm in diameter.

Preferably, nanoparticles have a diameter less than 50 nm, andpreferably under 30 nm. Ideally, these particles should be less than 15nm in diameter to maximize the surface interaction with platinum.

In another embodiment, the transition metals cobalt, iron, nickel,ruthenium, chromium, palladium, silver, gold and copper or alloysthereof comprise the nanoparticles or core, if an oxide shell ispresent. Although not being bound by theory, these elements acceptelectrons from platinum, which is preferable to observe the enhancedcatalysis. Alloy nanoparticles preferably comprise two or moretransition metals, or has two, three or four. The transition metalsspecified previously can be prepared in a variety of ratios to yieldperformance enhancement. The application in which the electrodes areused will dictate the alloy composition. In one embodiment, one metal ofthe alloy can range anywhere from 5 to 95% by weight of the alloy. Inone embodiment, one metal of the alloy is greater than 10% by weight, orgreater than 25%. In one embodiment, one metal is 90% by weight of thealloy.

In the composition, the nanoparticles are 5% or more by weight of thenanoparticles and platinum particles combined. In another embodiment,nanoparticles are 25% or more by weight of the nanoparticles andplatinum particles, or 50% or more by weight.

Preferably, at least 50% of the platinum by total metal weight ofconventional compositions is replaced with metal nanoparticles or metalalloy nanoparticles. The nanoparticles may also be 75% or more by weightor 90% or more by weight.

In another embodiment, the platinum/nanoparticle admix is combined withan ionomer, in many cases, a proton conducting ionomer, to promote ionicconductivity and to bind the electrode to a conducting membrane. Thisionomer may be combined with the platinum-mixture nanometal mixture andcan be up to 40% by weight of the total platinum and nanometal weight.The combination of platinum, nanometal particle, and ionomer forms anink. Preferably, the ionomer is a perfluorinated resin, which has bothhydrophobic and hydrophilic properties. More preferably theperfluorinated resin is a conducting polymer.

The ink composition may be used with an electron-conducting support toform an electrode. In one embodiment, this ink is applied to anelectrically conductive carbon substrate. The electron-conductingsupport may also be carbon paper, cloth, or powder. The ink compositionmay be applied to the electron-conducting support by painting, screenprinting, or spraying. The electrode subsequently may be applied to anion-exchange membrane and used in a direct oxidation fuel cell. Thisfuel cell is capable of converting chemical energy directly toelectrical energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron micrograph of cobalt metalnanoparticles.

FIG. 2 is a transmission electron micrograph of cobalt-nickel alloynanoparticles.

FIG. 3 details the cross-section of a direct oxidation fuel cell anodeor cathode electrode.

FIG. 4 shows a drawing of a direct methanol fuel cell.

FIG. 5 shows a voltammogram of cathode electrode performance.

FIG. 6 shows a voltammogram of cathode electrode performance.

MODES OF CARRYING OUT THE INVENTION

The inclusion of nanoparticles of metal, alloy and/or either having anoxide shell in the ink composition serves to improve the efficiency ofoxidation and reduction reactions by increasing the reaction surfacearea as well as enhancing electrocatalysis. The observedelectrocatalysis enhancement can be explained by molecular orbitaltheory. Since the nanoparticles are in good contact with platinum, theyaccept electrons from platinum. In turn, platinum becomes electrondeficient, and will react faster with the oxidant and reductant, therebyincreasing the efficiency of the reaction.

Due to increased surface area, when nanoparticles are blended withplatinum, water, and an ionically conducting polymer to form an ink, theactivity of platinum is increased due to enhanced contact of theplatinum and the nanoparticles. This contact serves two main functions,a) to enhance the electronic interaction of platinum with the oxidant orreductant by virtue of increasing the d-orbital vacancy on Pt by thenanoparticles, and b) to efficiently disperse Pt throughout the ink sothat it has improved contact with the oxidant and/or reductant.Additionally, metal alloy nanoparticles also provide these benefits. Ametal alloy nanoparticle is a compound which has individual metalcomponents combined in such a way such that combination gives thecompound unique chemical structure and properties in each individualparticle.

In this catalytic ink formula, the platinum particles should preferablybe small enough such that they can have strong surface interactions withthe nanoparticles. Preferably, the platinum should be finely divided.Platinum is considered to be finely divided when the particle size isbelow a micron, preferably below 500 nm in diameter such as from 1-500nm. Although finely divided platinum particles are adequate, it ispreferred that the platinum particles have a diameter below 100 nm tomaximize the platinum-nanoparticle surface contact. Preferred diameterof platinum particles are 1-100 nm, more preferably from 5-50 nm, mostpreferably from 5-25 nm.

Nanoparticles as used herein refer to metal nanoparticles, metal alloynanoparticles, or nanoparticles of metal or alloy having an oxide shellor mixtures thereof. Additionally, the individual nanoparticles shouldpreferably have a diameter below 50 nm, and preferably below 15 nm suchas from 1-15 nm. In initial studies, it was found that particles at themicron level do not exhibit the catalytic enhancing effect that thenanoparticles show. In studies using micron sized-metals and platinum inthe ink, a decrease in performance was observed due to lower surfacearea. Further the micron particles fall out of the electrode, andultimately lead to electrode failure. Thus, the high surface areananoparticles are necessary for proper electronic interaction anddispersion with platinum.

In addition, it is preferable that the metal or alloy nanoparticles havean oxide shell or outer surface, with a shell thickness of 1-25 nm, mostpreferably in the 1-10 nm range. These particles can be produced byvapor condensation in a vacuum chamber, and oxide thickness can becontrolled by introduction of air or oxygen into the chamber as theparticles are formed.

The nanoparticles that can be used in the ink may comprise a variety ofthe d-block transition metals, including cobalt, iron, nickel,ruthenium, chromium, palladium, silver, gold, and copper or mixturesthereof. Platinum is known to donate its electrons to these elements,thereby making platinum more reactive to the fuel.

Additionally, the nanoparticles can comprise two or more individualmetals, which form a metal alloy nanoparticle. The individual metals ofthe alloy can be combined in any ratio ranging from 5-95%. The ratio ofthe metals used in each particular alloy for the ink largely depends onthe catalytic application. The metal alloy nanoparticles representedhere can be two or more of the following transition metals cobalt, iron,nickel, ruthenium, chromium, palladium, silver, gold, and copper. Forexample, in a nickel/cobalt nano-alloy used in an electrode for a fuelcell operating at room temperature requires a higher content of cobaltin the alloy. For a room temperature direct methanol fuel cell, a 50:5060:40, 70:30, and 80:20 wt % ratio nanometal alloy of cobalt and nickelshowed the largest increase in electrical performance, because itefficiently accepts electrons from platinum. However, other ratios alsowork efficiently in conjunction with platinum. For a cathode electrode,a 50:50, 60:40, 70:30, and 80:20 wt % nanometal alloy of cobalt andsilver or cobalt and gold gives excellent electrical performance becausethe silver or gold component imparts increased methanol tolerance whilethe cobalt component improves oxygen reduction kinetics. Other ratiosalso work efficiently in conjunction with platinum. When palladium isalloyed with cobalt, nickel, iron, or silver in 50:50, 60:40, 70:30, and80:20 wt % ratios, catalytic enhancement is observed compared to pureplatinum for oxygen reduction. In higher temperature fuel cells, such asthe hydrogen PEM fuel cell, an 20:80 wt % ratio of cobalt to nickel ispreferred, which imparts greater stability due to the increased nickelcontent. However, other ratios also work efficiently in conjunction withplatinum. As an anode electrode, a 33:33:34 wt percent ratio ofchromium:ruthenium:platinum works to enhance the kinetic of methanoloxidation. In addition, a 50:50 chromium-ruthenium alloy used in 60 wt %ratio and 40 wt % ratio also shows performance higher than traditionalanode electrodes.

Along with platinum and the nanoparticles, an ink or catalyst inkcontains an ionomer which enhances physical contact between theelectrode and the fuel cell membrane, and also promotes ionicconductivity at the electrode-membrane interface. The most common typeof fuel cell membrane is the proton exchange membrane, in which case theionomer is proton conducting.

Preferably, the ink contains enough of the ionomer such that adhesion tothe membrane and ionic conductivity are enhanced, likewise, it ispreferred that the ionomer not be in excess of 40% by weight of thetotal ink. Preferably, the ionomer is present from 5-40% by weight oftotal metal loading, more preferably 10-30% and most preferably 15-25%.“Total metal loading” is total amount of metal in the ink. At highconcentrations of ionomer, a large resistance builds in the electrode,and blocks electrons from efficiently moving through the externalcircuit of the fuel cell.

The ratio of platinum to the nanoparticles will largely depend on themode of fuel cell operation. The catalyst blend is very sensitive tooxidant and reductant concentration and temperature. Due to the highcost of platinum, high nanoparticle fractions are ideal. A minimum of 5%nanoparticles (i.e., without platinum) by weight of total metal contentis preferred to observe increased catalytic activity, however over 90%of platinum by weight of conventional compositions can be replaced withthe nanoparticles. Most preferably, 50 to 75% of platinum particles arereplaced by metal and/or alloy nanoparticles.

In a direct oxidation fuel cell, such as the methanol fuel cell, theionomer conducts protons. A typical ionomer used in the ink is Nafion®,a perfluorinated ion exchange polymer. The polymer resin contains bothhydrophilic and hydrophobic domains such that there is a balance of bothwater-rejecting and water accepting properties. Although water providesimproved proton conduction, an excess of water blocks catalyst sitesfrom the oxidant and reductant, thereby lowering fuel cell efficiency.

The ink composition is prepared by mixing dry platinum and drynanoparticles in any ratio, such as those specified above. Preferably,several drops of water are added to the mixture to minimize the risk offire. Finally, the ionomer of specified amount is added, and theresulting ink is blended, for example, on a vortex mixer and sonicated,for example, for several minutes. The electrode is prepared bydepositing the ink on a conductive support. The conductive supportconducts electrons from the membrane-electrode interface to the fuelcell external circuit.

The ink is usually applied to the electron-conducting support by directpainting, spraying, or screen printing. The method chosen is notcritical to electrode performance in the fuel cell, however the methodshould preferably ensure an even coating of ink across an entire surfaceof the electrode.

The ideal material to use for the electron conducting support is carbon,however other electronically conducting materials can also work. Wovencarbon paper or fabric serves to support the ink, conduct electrons, andallow for the influx of oxidant and reductant by virtue of its porousnature.

In a direct oxidation fuel cell, the electrodes can be thermally pressedto either side of an ion conducting membrane. In the case of the directmethanol fuel cell, the electrodes can be applied onto a protonconducting polymer, for example by hot pressing, and subsequently placedin contact with bipolar plates that efficiently conduct electrons.

In the experiments below as presented by the data in FIGS. 1-6, thenanoparticles used have a metal core as indicated and have an oxideshell. The name of the metal without reference to the oxide shell isused for simplicity.

FIG. 1 shows a transmission electron micrograph image of nano-sizedcobalt particles that can be used in the ink. The average size of theseparticles are 8 nm, and their surface can come in excellent contact withfinely divided platinum. The level of contact between the platinum andmetal nanoparticles is directly quantified by the increase in catalyticenhancement observed from the oxidant/reductant reaction on the surfaceof the electrode.

FIG. 2 shows a transmission electron micrograph image of nano-sizednickel-cobalt alloy nanoparticles that can be used in the ink. Theaverage size of these particles is 12 nm, and their surface can come inexcellent contact with finely divided platinum. The level of contactbetween the platinum and nanoparticles is directly quantified by theincrease in catalytic enhancement observed form the oxidant/reductantreaction on the surface of the electrode.

FIG. 3 depicts the cross section of the fuel cell electrode (1). Thecatalyst ink (3) and the electron-conducting support (2) composed ofcarbon fibers (4). In the ink layer, platinum (5) and the nanoparticles(6) are in intimate contact with one another, and supported inside theionomer (7).

FIG. 4 depicts a direct methanol fuel cell (8). Aqueous methanol is fedinto the anode port (9), where it is circulated through port (10) orremains inside the cell. The methanol reacts at the anode electrode (11)(encompassing the ink (12) and the electron-conducting support (13)) toproduce carbon dioxide, protons, and electrons. Protons pass through theproton exchange membrane (14) to the cathode compartment, and electronsflow through the external circuit (15) and into the cathode. Air is fedinto the cathode port (16), where it reacts with electrons and protonsproduced from the anode on the cathode electrode (17) (encompassing theink (18) and electron-conducting support (13)) to produce water, whichis removed at the other cathode port (19).

As one example, FIG. 5 data shows a linear sweep voltammogram of thefuel cell cathode reaction, which depicts how current density, j,increases as voltage, V, decreases. The total metal loading in each inksample is 8 mg/cm². The greater the magnitude of the current increasesas voltage decreases, the better the performance of the catalyst ink.Curve A represents a fuel cell cathode catalyst ink containing finelydivided platinum and no nanoparticles. Curves B-D show the increasedperformance by removing some of the platinum and replacing it with 8 nmdiameter cobalt metal nanoparticles. As shown by replacing at least 50%by total metal weight of the platinum with cobalt metal nanoparticles,the current magnitude increase is larger than for the platinum-onlyelectrode ink. Although substituting 30% by total metal weight of theplatinum shows the largest current magnitude increase, greater weightfractions of cobalt metal nanoparticles also work well. It is clear incurves B-D that by adding these nanoparticles to the catalyst ink, bothoxygen reduction kinetics (shown in Region 1) and mass transport (shownin Region 2) are improved. In other types of fuel cell electrodes,greater than 50% of the platinum can be replaced with the nanoparticles,and preferably up to 95% by total metal loading weight can be replacedwith nanoparticles.

FIG. 6 also shows a liner sweep voltammogram of the cathode fuel cellreaction, showing performance increasing using a metal alloynanoparticle electrode. Total metal loading was 8 mg/cm² for eachsample. It illustrates the improved performance of a 60% platinum 40%nickel-cobalt metal alloy, with average nickel-cobalt metal alloyparticle size of 15 nm, electrode (curve B) versus a finely dividedplatinum electrode (curve A). Similar to the previous example usingmetal nanoparticles, the current magnitude increases greater withincreasing voltage for the metal alloy nanoparticle sample, both in thekinetic activation (Region 1) and mass transfer regimes (Region 2). Inaddition, a performance inhibiting effect is observed for the electrodecontaining 60% platinum 40% 800 nm average diameter cobalt particles byweight (curve C). This data illustrates the importance of usingnanoparticles, as particles at or above the micron size observablydecrease electrode performance due to the incompatible surface areas ofthe finely divided platinum, at or less than 100 nm and the microncobalt, in the 800-1500 nm size range.

Many other nanoparticles when admixed with platinum and made into anelectrode ink, also show this performance enhancement. For example, when10 to 50% by weight of total metal loading of finely divided 50:50atomic ratio platinum:ruthenium is replaced with 15 nm average diameterchromium metal nanoparticles and are used in an anode electrode ink,catalysis enhancement is observed for methanol oxidation. Preferably,the mixture will contain 50% chromium and 50% platinum:ruthenium byweight, and more preferably the mixture will be at least 70% chromiumand 30% platinum:ruthenium by weight. Most preferred is a 85% chromium15% platinum ruthenium mixture by weight. Total platinum:rutheniumloading can also be reduced at the anode by addition of 10 nm averageparticle size palladium nanoparticles. Preferably, the mixture willcontain 50% platinum:ruthenium and 50% palladium by weight, and morepreferably the mixture will be at least 70% palladium and 30%platinum:ruthenium by weight. Most preferred is a 15% platinum:ruthenium85% palladium mixture by weight. As another example, methanol oxidationrate is enhanced by replacement of 50% by weight of total metal loadingof platinum with 80:20 nickel-iron alloy nanoparticles that have anaverage diameter if 15 nm, preferably, the mixture will be at least 70%nickel-iron alloy nanoparticles and 30% platinum. Most preferably is a15% platinum 85% chromium mixture by weight. In both of these cases,other nanoparticles and other ratios of metal alloy nanoparticles worksufficiently compared to the reaction of finely dividedplatinum:ruthenium.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrative embodiments, andthat the present invention may be embodied in other specific formswithout departing from the spirit or essential attributes thereof. Thepresent embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

1. A composition comprising a) nanoparticles of at least one metal; b)nanoparticles of an alloy of two or more metals; c) nanoparticles havinga core of the at least one metal and an oxide shell; d) nanoparticleshaving a core of the alloy and an oxide shell; or e) mixtures thereof;in admixture with platimun particles.
 2. The composition of claim 1,wherein the platinum particles are finely divided platinum particles. 3.The composition of claim 2, wherein the platinum particles have adiameter of less than 100 nm.
 4. The composition of claim 1, wherein thenanoparticles have a diameter of less than 50 nm.
 5. The composition ofclaim 4, wherein the nanoparticles have a diameter of less than 30 nm.6. The composition of claim 5, wherein the nanoparticles have a diameterof less than 15 nm.
 7. The composition of claim 1, wherein the metal ormetals are selected from the group consisting of cobalt, iron, nickel,ruthenium, chromium, palladium, silver, gold, and copper.
 8. Thecomposition of claim 1, comprising wherein the nanoparticles comprisethe alloy.
 9. The composition of claim 8, wherein a first metal in thealloy is greater than 5% by weight of the alloy.
 10. The composition ofclaim 9, where the first metal in the alloy is greater than 10% byweight of the alloy.
 11. The composition of claim 10, where the firstmetal in the alloy is greater than 25% by weight of the alloy.
 12. Thecomposition of claim 11, where the first metal in the alloy is greaterthan 50% by weight of the alloy.
 13. The composition of claim 12, wherethe first metal in the alloy is greater than 90% by weight of the alloy.14. The composition of claim 1, wherein the nanoparticles comprise oneor two metals selected from the group consisting of cobalt, iron,nickel, ruthenium, chromium, palladium, silver, gold, and copper. 15.The composition of claim 1, further comprising an ionomer.
 16. Thecomposition of claim 15, wherein the ionomer is a proton-conductingionomer.
 17. The composition of claim 15, wherein the ionomer is lessthan 40% by weight of a combined weight of the nanoparticles and theplatinum particles in the composition.
 18. The composition of claim 1,wherein the nanoparticles are 5% or more by weight of a combined weightof the nanoparticles and the platinum particles in the composition. 19.The composition of claim 18, wherein the nanoparticles are 25% or moreby weight of a combined weight of the nanoparticles and the platinumparticles in the composition.
 20. The composition of claim 19, whereinthe nanoparticles are 50% or more by weight of a combined weight of thenanoparticles and the platinum particles in the composition.
 21. Thecomposition of claim 20, wherein the nanoparticles are 75% or more byweight of a combined weight of the nanoparticles and the platinumparticles in the composition.
 22. The composition of claim 21, whereinthe nanoparticles are 90% or more by weight of a combined weight of thenanoparticles and the platinum particles in the composition.
 23. Thecomposition of claim 15, wherein the ionomer is a perfluorinated resinhaving both hydrophobic and hydrophilic properties.
 24. The compositionof claim 23, wherein the perfluorinated resin is a conducting polymer.25. An electrode comprising the composition of claim 15, and anelectron-conducting support.
 26. A method of producing an electrodecomprising applying the composition of claim 15 to anelectron-conducting support.
 27. A fuel cell comprising the electrode ofclaim 25.